The commercial success of optical fiber telecommunications has fostered the growth of optical fiber sensing applications by providing a ready supply of low cost, high quality components and test equipment. Another enabling characteristic of optical fibers that facilitates their use in fiber sensing is ultraviolet (UV) radiation photosensitivity. This photosensitivity allows the alteration of the internal structure of a fiber waveguide. Modification of the waveguide can be employed for a number of useful purposes. One such useful purpose is to induce a periodic modulation of the refractive index along the fiber core to create a wavelength selective reflector called a fiber Bragg grating (FBG).
A fiber Bragg grating refers to regular, periodically spaced changes in the refractive index made in the core of an optical fiber. These periodic changes reflect a very narrow range of specific wavelengths of light passing through the fiber while transmitting other wavelengths. The period of the change or modulation determines that narrow band of reflected wavelengths. These reflections are small and add together in a phenomenon known as the Bragg reflection, where a single large reflection results from the coherent addition of many small reflections spaced a multiple of half the wavelength apart. The wavelength that is reflected by the fiber Bragg grating can be altered in two ways, altering the temperature or inducing strain in the section of the optical fiber containing the Bragg grating. Therefore, by monitoring the reflected wavelength, Bragg gratings can be used as strain and temperature gauges. In addition, the telecommunications industry has driven their development for use in wavelength division multiplexing, laser transmitter wavelength stabilization, fiber lasers and dispersion compensation.
Other desirable properties of optical fibers include excellent light transmission characteristics over long distances and the ability to fabricate such fibers in lengths of many kilometers. Since the optical fibers can communicate information, for example video, audio, or data, long optical fibers containing one or more fiber Bragg gratings can be used for remote measurement of engineering or environmental parameters. In the petroleum industry, for example, it is important to accurately measure environmental parameters, such as the pressure or temperature being experienced at a certain depth, using an optical fiber, or the strain on that optical fiber at a certain depth. For instance, while drilling, the drill bit may drill into a high pressure layer, and from at least a safety and environmental standpoint it is important to obtain accurate pressure information. Optical fibers can be used to communicate certain environmental and physical parameters from wells being drilled, as well as from already completed wells.
Previous systems using fiber Bragg gratings, however, require a high degree of manufacturing control to produce desired configurations and close correlations between the wavelength response of the fiber Bragg grating and the light source or laser. Therefore, a need remains for optical fibers that can measure engineering parameters and are easier to manufacture and provides a broadband wavelength response.